Every 64 hours, NASA’s Chandra X-ray Observatory follows a path
that dodges darkness, stretches one-third of the way to the Moon, and
has a more elliptical shape than most orbiting satellites.
Chandra’s unique orbit – selected by engineers at
NASA’s Marshall Space Flight Center in Huntsville, Ala. –
is a reflection of the unique requirements to ensure the world’s
most powerful X-ray telescope could deliver its groundbreaking images
to Earth.
Chandra began collecting unprecedented images of our universe in
August 1999. In just over two years, Chandra has found the most
distant X-ray cluster of galaxies, captured the deepest X-ray images
ever recorded and discovered a new size of black hole.
But before Chandra could achieve those firsts, “deepests”
and “farthests,” long before the observatory was launched,
engineers first had to determine precisely the best path for the
observatory to take.
“There were several challenges to overcome,” said Larry
Mullins, the aerospace engineer who led the team to determine
Chandra’s trajectory – or flight path. “One
challenge was the sheer height of the orbit needed.”
At its high point, two-hundred-times higher than the Hubble Space
Telescope, Chandra’s orbit takes it 75,000 miles from Earth, far
outside the belts of radiation that surround our planet. This
radiation – while harmless to life on Earth – can
overwhelm an X-ray observatory’s sensitive instruments.
During each 64-hour orbit, Chandra remains outside the radiation belts
long enough to take 55 hours of uninterrupted observations. To
achieve this unprecedented altitude for an orbiting satellite, the
Marshall engineers crafted an elliptical, or oblong, orbit.
“It would have been simpler to create a circular orbit, but at
that altitude, it was out of the question,” said Mullins,
“because at its highest point, Chandra flies about 75,000 miles
higher than the Space Shuttle can travel.”
A Space Shuttle flies as high as 350 miles from Earth. Even at its
closest approach to Earth, Chandra’s altitude is about 6,000
miles from Earth. Space Shuttle payloads, such as Chandra, destined
for altitudes above the Shuttle’s range, have rocket motors
attached. Fired after the spacecraft is a safe distance from the
Shuttle, these rocket motors propel the spacecraft to its final
destination.
The Chandra X-ray Observatory, with a rocket booster known as the
Inertial Upper Stage, and support equipment, was the largest and
heaviest payload ever launched by the Space Shuttle.
The wide variation between the observatory’s highest and lowest
point from Earth is the result of the elliptical orbit designed by the
Marshall Center trajectory team. To achieve this unusual orbit,
rocket boosters propelled the observatory to the required altitude.
“In its simplest terms, the concept is similar to a
sling-shot,” explained Russell Stone, an aerospace engineer in
Marshall’s Space Transportation Directorate.
But the implementation was anything but simple. It took two years
just to create the computer software that would enable the team to
predict the evolution of Chandra’s orbit over its expected
10-year lifetime.
Another challenge was ensuring Chandra had nearly uninterrupted access
to its power source – the Sun. Although the observatory has
three batteries that store power, the Sun is the sole source of power
to those batteries.
“The observatory’s battery life is two hours,” said
Stone. “That’s how long it can operate on the solar power
it’s collected, so Chandra can’t be in darkness for more
than two hours at a time. We had to find a path that minimized its
time in Earth’s shadow.”
“We did some parameter studies and found the size and
orientation of an orbit that fit all these criteria,” said Steve
Evans, another member of the Chandra Trajectory team at the Marshall
Center. In fact, the engineers don’t expect Chandra to go into
a shadow for more then two hours at a time for the next 10 years.
“Chandra’s orbit is so high that it doesn’t move
into Earth’s shadow for every one of its orbits,” Evans added.
In fact, it is infrequent enough to result in only two eclipse seasons
a year, each with about a dozen eclipses.
Based on the observatory’s outstanding results, in September
2001 managers at NASA Headquarters in Washington, D.C., decided to
extend Chandra’s mission from its original five-year-mission to
a 10-year mission.
The extended mission will support five additional years of day-to-day
operations such as controlling the spacecraft, observing celestial
targets, processing the data, and passing it on to scientists around
the globe. It also includes continuing the administration of hundreds
of science grants for astronomers to analyze their data and publish
their results.
What Chandra will discover during its additional five years remains to
be seen, but thanks to the efforts of NASA engineers, there’s no
mystery to the path the observatory will take while delivering its
groundbreaking images to stargazers on Earth.
